Coating for imparting non-stick, abrasion resistant and non-wetting properties to inorganic articles

ABSTRACT

A protective coating which imparts non-stick and abrasion resistant properties, as well as hydrophobic properties to an inorganic article, the coating being the composition product between hydroxyl groups on the surface of the inorganic article and a composition comprising a silane, a metal alkoxide and an acidified aqueous mixture of a lower alkanol. The silane content is at least 50% of the total sum of the silane plus the metal alkoxide. The metal alkoxide is at least 5% and preferably 10% of the total sum of the silane plus the metal alkoxide.

RELATED APPLICATION

An application entitled SYSTEMS FOR IMPARTING NON-STICK AND NON-WETTINGPROPERTIES TO SURFACES, filed under Ser. No. 60/007,704 in the names ofA. Carre, M. Prassas and J. Waku and assigned to the same assignee asthis application, is directed to a material system and method forimparting non-stick and non-wetting properties to an inorganic articlehaving hydroxyl groups on its surface, the system comprising a silanehaving the formula R_(n) MX_(4-n) wherein n is 1 or 2 and X is ahydrolyzable group, a colloidal metal oxide sol, and an acidifiedaqueous mixture with a lower alkanol.

The present application claims the benefit of French Application 9511373, filed Sep. 28, 1995 (28-09-1995), and of U.S. ProvisionalApplication Ser. No. 60/007,705, express mailed Nov. 29, 1995, entitledSYSTEMS FOR IMPARTING NON-STICK AND NON-WETTING PROPERTIES TO SURFACES,by A. Carre, N. Prassas and J. Waku.

FIELD OF THE INVENTION

The field is a material system and a process for coating articles,particularly glass and glass-ceramic articles, to impart durable,non-stick and non-wetting properties

BACKGROUND OF THE INVENTION

Considerable effort has been directed at producing an effective,non-stick and non-wetting coating on the surface of an inorganicarticle, in particular a utensil for preparing and/or serving food. Theterm "non-stick" signifies a surface that resists adherence of foreignsubstances, such as food. This property permits easy cleaning of dishesin which foods are cooked or baked. The term "non-wetting" indicates asurface that repels liquids such as water. The property is evidenced bya large contact angle between a drop of water and the surface on whichthe drop rests. An advancing angle of at least 90° is considered asrepresentative of a non-wettable surface.

Non-stick, non-wetting articles are commonly produced by coating articlesurfaces with a polytetrafluoroethylene (PTFE) layer. For example, U.S.Pat. No. 4,683,168 (Hares et al.) describes a process for coating glassor glass-ceramic articles with such a layer to produce non-stick cookingutensils. However, PTFE coatings exhibit the disadvantage of beingopaque. Also, they require a thermal sintering process at 350°-400° C.for their production, are relatively expensive to produce, and do notprovide a desired degree of abrasion resistance.

French Patent No. 2674862 (Carre) discloses an alternative materialsystem and process for producing durable, non-stick and water repellentcoatings on glass and glass-ceramic surfaces. The process involvesapplying a mixture of a selected silane with a lower alkanol and waterwhich may be acidified. The silane has the formula R_(n) SiX_(4-n),where R is an alkyl radical and X is a hydrolyzable alkoxy or chlorogroup. The mixture is applied to a surface containing hydroxyl groups,the silane is hydrolyzed and the alkanol-water mixture eliminated toform a polysiloxane coating adhering to the surface.

Numerous other processes and material systems have been proposed forrendering inorganic surfaces, particularly vitreous surfaces,non-sticking and water repellent. The desired properties can be obtainedinitially. However, there has persisted a problem with respect to wearduring use, in particular, scratching by cleaning agents. It is, then, apurpose of the present invention to improve on the Carre process bvimparting a greater abrasion resistance to the coating there disclosed

SUMMARY OF THE INVENTION

The present invention resides in part in a material system for impartingnon-stick and non-wetting properties to an inorganic article havinghydroxyl groups on its surface, the system comprising a silane havingthe formula R_(n) SiX_(4-n) wherein n is 1-2 and X is a hydrolyzablegroup, a metal alkoxide having at least three hydrolyzable branches anda formula R MX₃ or MX₄ wherein M is a metal and X is a hydrolyzablebranch, and an acidified aqueous mixture with a lower alkanol.

The invention further resides in a method of imparting non-stick andnon-wetting properties to an inorganic article having hydroxyl groups onits surface which comprises mixing a silane having a formula R_(n)SiX_(4-n) l, with a metal alkoxide having a formula R MX₃, or MX₄,adding an acidified aqueous mixture of a lower alkanol, applying acoating of the mixture to a surface on the inorganic article, thermallyprocessing the coating at a temperature of at least 100° C. to form asolid, transparent film on the article surface

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A graphical illustration showing contact angle as a function ofmol % of dethoxy dimethyl silane (DEDMS).

FIG. 2 A graphical illustration showing contact angle as a function ofthe ratio of acid to the sum of silane of FIG. 1 plus alkoxide.

DESCRIPTION OF THE INVENTION

The present invention arose from studies directed at improving thepolysiloxane, non-stick coating disclosed in French Patent No. 2674862(Carre). The non-stick properties of the Carre coating are adequate.However the coating, like other known non-stick coatings, is prone todamage and removal. This is particularly true when the coating issubjected to the abrasion occasioned by normal scrubbing. Accordingly,efforts were directed at improving the abrasion-resistance of the knowncoating while either retaining, or improving on, its non-stickcharacteristics.

We have now found that the desired improvement can be achieved byincorporating a selected metal alkoxide with the prior silane componentto achieve a hybrid coating. The coating retains the good non-stick andnon-wetting characteristics of the prior polysiloxane coating. It addssubstantially better thermal stability and resistance to detergent andabrasion treatment.

The new hybrid coating material, like the earlier silane coatingmaterial, is effective when applied to an inorganic article havinghydroxyl groups on its surface, in particular, a glass or glass-ceramicarticle. The silane and the metal alkoxide are hydrolyzed to producehydroxyl groups. These groups, produced during hydrolysis, link with thehydroxyl groups on the article surface to create a surface exhibitinghydrophobic properties.

The coating material represents a mixture of at least three components:(a) a silane having the formula R_(n) SiX_(4-n), (b) a metal alkoxidehaving the formula R MX₃ or MX₄, and (c) a mixture of a lower alkanoland acidified water.

The silane having the formula R_(n) SiX_(4-n) is a silane wherein each Ris chosen independently from among the methyl, ethyl, and propylradicals, unsubstituted or partially or totally substituted by fluorineatoms; X is a hydrolyzable group chosen from among the methoxy, ethoxy,and chloro groups; and n=1 or 2. A mixture of a lower alkanol and wateris added to the silane constituent as a solvent. The water is preferablyacidified, at least where X is a methoxy or ethoxy group.

Among the usable silanes encompassed within the above formula, it ispreferred that R be a non-substituted lower alkyl-radical, the methylgroup being the most preferred. The non-substituted silanes arepreferred from a practical point of view. They are more economical thansilanes containing fluorinated radicals.

The silane component can be formed exclusively of a silane having theabove formula where n=2 or 1. It may also be a mixture of silanes havingthe above formula where n=2 and where n=1. In a mixture, the silanehaving the above formula where n=2 preferably constitutes at least 20mole percent of the silane mixture. The most preferred silane of theabove formula where n=2 is dimethyldiethoxysilane. The most preferredsilane of the above formula where n=1 is methyltriethoxysilane.

The alkanol employed in the alkanol-water mixture can be a lower alkanolsuch as methanol, ethanol, or a propanol such as isopropanol. Ethanol orpropanol is preferred, the choice depending on the method ofapplication. The role of the alkanol is simply to permit the solutionand/or the dispersion of the silane in the aqueous mixture, the silanenot being soluble in water.

Where a non-chlorinated silane constituent is used, the alkanol-watermixture can conveniently contain 5-93% by volume water, preferably 5-60%by volume. In contrast, where a chlorinated silane component isemployed, the proportion of water in the alkanol-water mixture can bereduced to a very small amount, i.e., to the order of trace amounts.

The water is acidified with a mineral or organic acid, for example, toreduce the pH below approximately 4 where an alkoxylated silaneconstituent is used. Examples of operable acids include hydrochloricacid, nitric acid, sulfuric acid, and acetic acid. When a chlorinatedsilane constituent is utilized, it is not always necessary topre-acidify the water. It is acidified automatically as soon as thesilane hydrolysis commences as a result of the formation of HCl.

The proportion of silane in the composition is not very critical. Forexample, it may be about 0.06-1 mole/liter, preferably about 0.1-0.6mole/liter, of the mixture.

The metal alkoxide component of the coating mixture has a formula R MX₃or MX₄. This provides an alkoxide with at least 3 hydrolyzable branchesto increase cohesion and bonding of the components. This provides athickened layer and increases the links with the inorganic surface toenhance the coating.

In the alkoxide formula, M is a metal, preferably selected from thegroup consisting of silicon, titanium, zirconium and aluminum. R, ifpresent, may be a methyl, ethyl, propyl, or butyl group. In preparing acoating material, the mixture of alkanol and water is added to thesilane as a solvent therefore. If needed to allow the silane to undergohydrolysis, acid is then added. The mixture is continuously stirred toprovide a homogeneous mixture. The metal alkoxide is then added whilemaintaining an inert, e.g. N₂, atmosphere. This is necessary becausemost alkoxides are quite moisture sensitive, even at ambient, and tendto undergo hydrolysis.

The coating material is now applied to the surface to be protected. Thehydrolyzed X groups on the silane and alkoxide components can react bycondensation with the hydroxyl groups present at an inorganic surface toform stable bonds with that surface. The overall result of thesereactions is the formation of a thin, hybrid coating. This coating isdurably bonded to the surface of the treated inorganic article andconfers non-stick and water repellent properties to that surface. Theinventive compositions for treating inorganic surfaces exhibit limitedstability and, therefore, must be used within a reasonably short time oftheir preparation.

The reaction of the hydrolyzed groups with the hydroxyl groups on aninorganic surface can be carried out at ambient temperature. However,that reaction is rather slow (about 24 hours). The time required may besubstantially shortened by a heat treatment, for example, about 80° C.

To complete densification, the coated article may be heated to atemperature of about 100°-400° C. for a period of time. This can be, forexample, about 5-30 minutes, depending upon the particular compositionused for the treatment and upon the temperature of heating.

One method of forming the protective coating comprises briefly dippingthe article to be treated, while it is at an elevated temperature, e.g.,100°-300° C., into a bath of the inventive composition for a fewseconds. The article is then permitted to cool naturally. Theevaporation of the alkanol-water mixture and the condensation reactionstake place during that cooling.

This mode of forming the desired coating is especially economical andconvenient in operation for glass and glass-ceramic articles. Themanufacturing of such articles customarily involves heating processes,e.g., in forming the article, or in annealing, tempering, ceramming, orother forms of thermal treatment. With such articles, the inventiveprocess can be implemented in a production line. The article will be ata temperature sufficiently high to be suitable for carrying out thethermal treatment necessary for the formation of the coating.

Other methods of forming the protective coating involve painting,spinning, or spraying the coating material onto an unheated surface. Thematerial is then heated at 100-300° C. for 5-30 minutes to removevolatiles and set the coating.

The coating obtained from the inventive compositions imparts non-stick,hydrophobic, and water repellent properties to the coated surface. Theseproperties are surprisingly superior to those that can be obtained fromprior compositions based upon silanes alone. In particular, the presentcoatings are much more resistant to detrimental effects from abrasivesand detergents.

The nature of the protective coating, and its production, may be seen inthe following projected formulations:

a. A glass or glass-ceramic surface containing hydroxyl groups. ##STR1##b. Hydrolysis of a hydrolyzable silane ##STR2## c. Hydrolysis of a metalalkoxide ##STR3## d. The coated, hydrophobic surface ##STR4##

The hydroxyl groups produced by hydrolysis in (b) and (c) link withthose on the article surface (a) to form a hydrophobic coating (d). Thisimparts hydrophobic properties to the surface.

The presence of acid in the coating material is necessary to permithydrolysis of the silane and alkoxide. To this end, the ratio of acid tothe sum of silane plus alkoxide should be at least 0.4, and preferablyin the range of 0.6 to 5.2.

Silanes and metal alkoxides have different hydrolysis rates. A smallamount of a known complexant molecule may be employed to control theserates. This achieves a desirable structure in the hydrolyzed coating andrenders it clear and transparent. Diketonates, p-diketonates andcarboxylic acids are among the complexants that may be used.

Choosing a ratio of silane to alkoxide in the coating materialrepresents a compromise. The silane, as observed earlier, provides goodnon-stick properties. These are evidenced by high contact angles.However, the coating does not have good resistance to abrasion anddetergent attack.

We have now found that the presence of certain metal alkoxides in asilane coating material markedly improves the abrasion and detergentresistance of a coating However, the metal alkoxide alone does notprovide a coating having non-stick characteristics. Accordingly, it isnecessary that the silane content in a coating material be at leastequal to the metal alkoxide content. Preferably, the silane is inexcess.

The effect is illustrated in FIG. 1 of the accompanying drawing ThatFIGURE is a graphical illustration wherein contact angle in degrees isplotted on the vertical axis. The mol percent of silane with respect tosilane plus alkoxide is plotted on the horizontal axis. The silane wasdiethoxy dimethyl silane (DEDMS) and the alkoxide was zirconium (IV)propoxide (Zr[pro]₄) in mixtures employed to obtain the plotted data.

It will be observed that the contact angle falls off rapidly as thesilane content becomes less than 50% of the total silane plus alkoxide.However, at least 5%, and preferably at least 10%, alkoxide is usuallynecessary to provide a substantial improvement in abrasion and detergentresistance.

The presence of an acid is necessary to permit hydrolysis of the silaneand alkoxide. To this end, the ratio of acid to the sum of silane plusalkoxide should be at least 0.4 and preferably at least 0.6 and stillmore preferably at least 1.2 This is illustrated in FIG. 2 of thedrawing. Again, contact angle, in degrees, is plotted on the verticalaxis, while the ratio of acid to the sum of silane plus alkoxide isplotted on the horizontal axis.

It will be observed that the contact angle becomes insufficient as theratio falls below about 0.4, but is reasonably stable at higher ratios.The acid is HNO₃ or HCl, the alkoxide is zirconium propoxide, and thesilane is diethoxy dimethyl silane

SPECIFIC EMBODIMENTS

TABLE I sets forth a series of exemplary compositions that illustratematerial systems in accordance with the invention. The silane employedin each composition was diethoxydimethyl silane (DEDMS). The alkanolselected was ethanol and the acids employed were 12M HCl and 15M HNO₃.Both are shown in moles/liter (mol/l).

The metal alkoxides were zirconium (IV) propoxide, shown as Zr(pro)₄ ;tetraisopropyl-orthotitanate, shown as TIOT; and aluminum-sec-butoxide,shown as Al(but)₃. Both the silane and alkoxide contents are shown inmoles/liter (mol/l).

The several compositions were applied to a glass-ceramic surface byspinning. The coated samples were extensively tested for contact angleagainst water after various treatments. Measurements were made on testpieces after (1) an initial thermal treatment at 120° C. for one hour toset the coating (θ), (2) after being held at 360° C. for 30 minutes(360°/30); (3) after 30 minutes at 450° C. (450°/30); (4) after exposureto a detergent, Super Soilax®, for 8 hours at 95° C. (DT).

                  TABLE I                                                         ______________________________________                                        Component 1      2        3    4     5     6                                  ______________________________________                                          DEDMS (mol/l) 0.4 0.4 0.4 0.4 0.4 0.4                                         Zr(pro).sub.4 (mol/l) 0.3 0.3 0.3 0.3 0.3 0.3                                 HCl (mol/l) 0.9 1.2 1.5 -- -- --                                              HNO.sub.3 (mol/l) -- --  1.2 1.5 1.9                                          ethanol (mol/l) 12.8 12.5 12.2 12.8 12.5 12.2                                 θ 96 100 91 103 99 90                                                   360°/30 96 96 91 96 90 90                                              450°/30 96 94 91 86 80 70                                              DT 88 78 82 98 80 73                                                        ______________________________________                                        Component 7      8        9    10    11    12                                 ______________________________________                                          DEDMS (mol/l) 0.4 0.4 0.4 0.4 0.4 0.4                                         Zr(pro).sub.4 (mol/l) -- -- -- -- 0.14 0.1                                    TIOT (mol/l) 0.2 0.2 0.2 0.2 0.2 0.133                                        Al(but).sub.3 (mol/l) -- -- -- -- -- 0.133                                    HCl (mol/l) 1.0 1.5 -- -- -- --                                               HNO.sub.3 (mol/l) -- -- 1.2 1.5 1.9 1.9                                       ethanol (mol/l) 13.6 13.2 12.9 13.2 12.9 12.9                                 θ 93 90 98 92 97 100                                                    360°/30 86 90 75 65 80 97                                              450°/30 50 90 62 63 36 42                                              DT 86 90 90 70 83 85                                                        ______________________________________                                    

Compositions 1, 4 5 and 6 are preferred because of their stability, theconstancy of their performance under various conditions and ability towithstand thermal processing Composition 4 was subjected to variouspunishing treatments after being applied to commercial glass-ceramicarticles having a lithium aluminosilicate (beta-spodumene) crystalphase.

Each coated article was evaluated for food release using a scale of 0-5with 5 indicating excellent release and 0 indicating no release. Theresults were

    ______________________________________                                        Treatment        θ                                                                              Food Release                                          ______________________________________                                        Uncoated         35     0                                                       Coated 103  5                                                                 Detergent 98 4                                                                8 hours/95° C.                                                         4% Acetic acid 75 4                                                           48 hours/95° C.                                                      ______________________________________                                    

A coated article, otherwise untreated, was subjected to a scrubbingaction with a commercial cleansing pad. After 100 rotations at apressure of 0.25 Kg/cm² (3.5 lbs/inch²), food release was consideredgood, after a thousand rotations, release was still considered adequate.

EXAMPLE 13

The following material composition, also in mol/l, illustrates use ofacetic acid as a complexant and of 2-propanol as a solvent:

    ______________________________________                                               Component                                                                             Content                                                        ______________________________________                                               DEDMS   0.64                                                             Zr(pro).sub.4 0.44                                                            HNO.sub.3 2.16                                                                H.sub.2 O 3.33                                                                Acetic acid 0.03                                                              2-propanol 2.20                                                             ______________________________________                                    

A mixture of these components was prepared and applied to aglass-ceramic surface by spraying. The sprayed article was cured for 30minutes at 250° C. Surface finish, food release and durability werefound to be excellent. The non-stick properties were not affected byeither heat treatment at 360° C. for 30 minutes or by exposure to SuperSoilax at 95° C. for 8 hours.

EXAMPLE 14

The following material composition, expressed in mol/l, illustrates theuse of dimethyl dichlorosilane.

    ______________________________________                                               Ingredient                                                                            Content                                                        ______________________________________                                               DMDCS*  0.33                                                             Zr(pro).sub.6 0.06                                                            H.sub.2 O 0.33                                                                Acetic acid 0.02                                                              Alkanol 15.90                                                               ______________________________________                                         *dimethyl dichlorosilane                                                 

A mixture of these ingredients has been prepared and coated on aglass-ceramic surface by spinning. After a curing of 250° C. for 30minutes, the contact angle was 116°.

We claim:
 1. A protective coating for an inorganic article providesimproved resistance to abrasion, resists adhesion to foods and ishydrophobic, the coating being the condensation product between hydroxylgroups on the surface of the inorganic article and a compositioncomprising the following components:(a) a silane having the formulaR_(n) SiX_(4-n), whereineach R is chosen independently from the groupconsisting of methyl radical, ethyl radical, or propyl radical,unsubstituted or partially or totally substituted by fluorine atoms; Xis a hydrolyzable group chosen from the group consisting of methoxy,ethoxy and chloro groups; and, n=1 or 2; (b) a metal alkoxide having atleast three hydrolyzable branches and a formula R MX₃ or MX₄, whereinRis selected from the group consisting of methyl group, ethyl group,propyl group, and butyl group, M is a metal selected from the groupconsisting of silicon, titanium, zirconium and aluminum, and, X is ahydrolyzable group selected from the group consisting of methoxy, ethoxyand chloro groups; and, (c) a mixture of lower alkanol and acidifiedwater, the water being acidified at least in cases where X is a methoxyor ethoxy group; wherein(i) the silane content is at least 50 mole % ofthe total sum of the silane plus the metal alkoxide; (ii) the metalalkoxide is at least 10 mole % of the total sum of the silane plus themetal alkoxide.
 2. The protective coating of claim 1 wherein the metalalkoxide is zirconium propoxide.
 3. The protective coating of claim 1wherein the inorganic article is a glass or glass-ceramic.
 4. Theprotective coating of claim 1 wherein the silane is diethoxy dimethylsilane.
 5. The protective coating of claim 1 wherein the lower alkanolis ethanol.
 6. A method of imparting non-stick and non-wettingproperties, and improved abrasion resistance, to an inorganic articlehaving hydroxyl groups on its surface, the method comprising thefollowing steps:(1) making a mixture of a silane having a formula R_(n)SiX_(4-n), with a metal alkoxide having a formula R MX₃ or MX₄,whereinin the silane each R is selected independently from the group consistingof methyl radical, ethyl radical, or propyl radical, unsubstituted orpartially or totally substituted by fluorine atoms; X is a hydrolyzablegroup selected from the group consisting of methoxy, ethoxy and chlorogroups; and, n=1 or 2,wherein in the metal alkoxide R is selected fromthe group consisting of methyl group, ethyl group, propyl group, andbutyl group, M is a metal selected from the group consisting of silicon,titanium, zirconium and aluminum, and, X is a hydrolyzable groupselected from the group consisting of methoxy, ethoxy and chloro groups;wherein(i) the silane content is at least 50 mole % of the total sum ofthe silane plus the metal alkoxide; (ii) the metal alkoxide is at least10 mole % of the total sum of the silane plus the metal alkoxide; (2)adding an acidified aqueous mixture of a lower alkanol to hydrolyze thesilane/metal alkoxide mixture, the water being acidified at least incases where X is a methoxy or ethoxy group; (3) applying a coating ofthe hydrolyzed mixture the surface of the inorganic article, the surfacecontaining hydroxyl groups, (4) eliminating the alkanol-water mixtureand allowing or causing a condensation reaction to take place betweenthe product of the hydrolysis of silane and metal alkoxide componentswith the hydroxyl groups on the surface of the inorganic article to forma coating which is bonded to the surface of the inorganic article. 7.The method of claim 6 wherein the coating is applied by first heatingthe inorganic article at a temperature of 100-300° C. and then dippingthe heated article in the mixture.
 8. The method of claim 6 wherein thecondensation reaction is caused by thermally processing the coating onthe inorganic article at a temperature of 100-400° C. for 5-30 minutes.